1. Field of the Invention
This invention concerns nuclear radiation dose rate meters utilizing a plastic scintillator material wherein the high sensitivity of a scintillator is combined with the low loss propagation capability of an optical fiber.
2. Description of the Prior Art
The darkening of optical materials upon exposure to ionizing radiation is a well-known phenomenon. The highest degree of darkening is found in lead-silicate fibers. It can be concluded that radiation induced color changes and their attenuating affects on light transmission are large at short wavelengths and decrease monotonically to very small affects at larger wavelengths.
The formation of radiation-induced color centers in optical fibers creates darkened or colored areas which give rise to not only increased light attenuation but also to decreased luminescent light generation within such fibers. The most extensively studied effect of ionizing radiation on optical fibers has been signal attenuation due to the production of defect centers which absorb the light transmitted by the fibers.
The spectral character of the optical absorption produced by steady-state and pulsed ionizing radiation in all types of optical fibers has been investigated by many researchers. It can generally be concluded that the most radiation-resistant fibers, that is, resistant to the formation of color defect centers, are formed of high purity synthetic silica, highly doped silica, cerium protected silicate glasses and plastics such as polystyrene.
The high radiation sensitivity of lead-doped silicate fibers has been used to advantage for radiation dosimeters using an optical fiber as a sensing element. Glasses have been successfully employed in block-form as radiation dosimeters for high dose levels. However, the poor optical quality and limited size of the glass block dosimeters have restricted the application of these devices.
Glass optical fibers greatly reduce the problems encountered with bulk glass dosimeters. A combination of low intrinsic optical loss and a greatly increased optical path length permit enhanced radiation sensitivity in optical fibers of at least two orders of magnitude compared to conventional glass block dosimeters. In addition, because of the flexibility and small size of the fibers, the sensing element can be confined to a very small volume.
Dosimeters are based either on a highly sensitive transmission measurement of the fiber or are based on comparing the color of two fibers in a personal dosimeter. Also, radiation induced luminescence has been suggested for application in radiation dosimeters.
Many solid and liquid substances show luminescence when exposed to ionizing radiation. The most important groups of scintillators in use today are the organic crystals of the hydrocarbon type, the inorganic crystals and powders of the alkali halide and zinc sulfide types, and liquid and plastic solutions with most hydrocarbon solutes. Noble gas and glass scintillators have also been investigated.
Scintillators that are of particular value for nuclear physics applications are those that exhibit transparency for the emitted light, with correspondingly high gamma and X-ray sensitivity and fast response. High sensitivity results in increased scintillation and light generation at such wavelengths. From the group of inorganic crystals, sodium iodide is the most efficient scintillator for the detection of X-radiation and gamma radiation known today. Liquid and plastic scintillators, although not quite as efficient as NaI, have a number of advantages.
A category of useful scintillators is produced by dissolving an organic scintillator in an appropriate solvent. Liquid scintillators are often sold commercially in sealed glass containers and are handled in the same manner as solid scintillators. In certain applications, large-volume detectors with dimensions of several meters may be required. Liquid scintillators are also widely applied to count nuclear radiation emissions associated with materials which can be dissolved as part of the scintillator solution. The technique is widely used for counting low level beta activity, such as that from carbon-14.
If an organic scintillator is dissolved in a solvent which can be then subsequently polymerized, the equivalent of a solid solution can be produced. The most common example is a solvent of styrene monomer or polyvinyltoluene (PVT) in which an appropriate organic scintillator, such as 2,5 diphenylloxazole (PPO) is dissolved. The styrene or PVT is then polymerized to form a solid plastic.
Because of the ease with which plastics can be shaped and fabricated, they have become an extremely useful form of organic scintillator. Plastic scintillators are available commercially with a good selection of standard sizes of rods, cylinders, and flat sheets.
Scintillators emit radiation as a result of gamma photon flux and, therefore, they are primarily used with dose rate counters. The most important interactions which can occur in a scintillator is the absorption of gamma photons. The Compton effect, which describes the elastic collision between a gamma ray photon and an electron of the scintillator material, is one type of such interaction. The photo-electric effect, wherein a gamma photon incident on an atom is absorbed and a photo-electron is ejected from the atom, is another type of such interaction. Yet another type of such interaction is pair production, where the energy of an incident gamma-photon exceeds 2m c=1.02 MeV. The photon is completely absorbed and the energy is converted into rest energy 2mc and kinetic energy of an electron-positron pair.
Conventional scintillation counters include one or several photomultiplier tubes directly attached to the scintillator material. The scintillator material may be a crystal, liquid or plastic housed in a light-tight enclosure.
Radiation sensitive fibers are known as having an inner core of glass and a cladding of plastic (U.S. Pat. No. 4,415,810) or an inner core of plastic and a cladding of glass (U.S. Pat. No. 4,467,208).
Typically, for a fiber having a glass core and a plastic cladding, a resin, polymer or monomer, etc., is applied in liquid form to a glass filament. The plastic is then quickly cured by heating. Because the glass core has a very high melting temperature, compared to the plastic, this curing process of the cladding has a detrimental effect on the glass.
Furthermore, glass is chemically inert and much harder than the cladding. The higher melting temperature, chemical inertness and relative hardness make the application of a plastic cladding tedious and produces a resulting fiber having low sensitivity to nuclear radiation. And, a fiber with a glass core and a plastic cladding is most commonly used as an optical fiber for communication purposes. Susceptibility to nuclear radiation is an undesirable side effect. Some glasses are more susceptible to nuclear radiation than others, but even the most sensitive glasses are many more orders of magnitude less sensitive compared to plastic scintillator materials.
And, a fiber with a plastic core and a glass cladding may meet the sensitivity requirement, but only short pieces can be made in a labor-intensive hand operation. Actually, only a few liquid filled capillary glass tubes are known to have been produced in a laboratory. A plastic core material requires forcing the polymer into the glass tube before polymerization. It is impossible to do this with capillary tubes which are smaller than 1 mm in diameter.